This invention generally relates to a rodless cylinder apparatus which permits movement of a piston within a cylinder tube without employing a piston rod, and more particularly to a rodless cylinder apparatus which is capable of positioning the piston at a desired position in its stroke within the cylinder.
As most typically represented by an air cylinder, cylinders are widely used as an actuator for positioning various parts and jigs placed on a table at a selected or desired position. In general, each of the cylinders comprise a cylinder tube, a piston provided within the cylinder tube for reciprocating movement therealong, and a piston rod for transmitting the reciprocating movement of the piston to an external element.
It is known that a cylinder having a piston rod requires a cylinder tube which is at least as long as the stroke over which the piston rod makes a reciprocating movement. This presents a problem that the longer the stroke length is, the bigger space is needed for installation of the cylinder.
Because of the above-noted problem, a rodless cylinder has become more popular in recent years which allows a piston to make a reciprocating movement without employing a piston rod. Having no piston rod, such rodless cylinder can be installed in much smaller space as compared with the traditional cylinders having a piston rod.
FIGS. 7, 8 and 9 illustrate an example of the prior art rodless cylinder in trigonometry: FIG. 7 is a plan view of the rodless cylinder as viewed in the Z-axis direction; FIG. 8 is a side-elevational view of the rodless cylinder as viewed in the Y-axis direction, and FIG. 9 is a side-elevational view of the rodless cylinder as viewed in the X-axis direction (that is, the piston movement direction). In FIGS. 8 and 9, the rodless cylinder is shown partly in cross section.
A cylinder tube 1a of the cylinder is in a rectangular parallelepiped shape, and it has a hollow interior portion that serves as a guide along which piston 2a makes an reciprocating movement. The cylinder tube 1a also has a longitudinal gap of a given width which extends in the axial direction of the tube 1a along the entire stroke, namely, length of the reciprocating movement of the piston 2a, in order to allow a piston yoke 2b to project outwardly of the tube 1a and to move freely along the tube 1a. Because of the gap, the cylinder tube 1a is generally C-shaped in cross section as viewed in the piston movement direction (X-axis direction). The gap in the cylinder tube 1a is sealed by a sealing belt 5 for fluid tightness in the tube 1a.
The piston 2a is composed of right and left pistons 2a each of which is a cylindrical column that corresponds in cross-sectional shape to the hollow interior portion of the cylinder tube 1a. The piston yoke 2b has an upper portion projecting outwardly of the cylinder tube 1a and terminating in a support portion for supporting thereon a table or article carrier 6. Within the cylinder tube 1a, the pistons 2a are coupled to the left and right side of the yoke 2b respectively. A piston packing 3 is provided on and around the outer circumferential surface of the piston 2a. In the piston yoke 2b, a slot 4 is provided through which the sealing belt 5 extends. The sealing belt 5 is movable relative to the yoke 2b along the slot 4, so that the yoke 2b is capable of making a free reciprocating movement with the piston 2a while fluid tightness in the tube 1a is maintained by means of the sealing belt 5.
The table 6 has a channel-shaped cross section and mounted on the upper support portion of the piston yoke 2b. The piston yoke 2b has in its upper surface a guide channel for a dustproof belt 7. The belt 7 is slidable along the guide channel of the piston yoke 2b between the table 6 and piston yoke 2b. The belt pressing member 8 is rotatably mounted about a shaft 9 and normally urged by a spring 10 for pressing the dustproof belt 7 against a wall defining the gap of the cylinder tube 1a.
End caps 11L and 11R are provided at opposite ends of the cylinder tube 1a and have air supplying tubes 11a and 11b, respectively, for supplying pressurized air into the cylinder tube 1a. Belt covers 12L and 12R are provided for fastening the dustproof belt 7 and sealing belt 5 at the opposite ends of the cylinder tube 1a. The end cap 11L, cylinder tube 1a, sealing belt 5 and left piston 2a together form a left-side chamber space, while the end cap 11R, cylinder tube 1a, sealing belt 5 and right piston 2a together form a right-side chamber space.
When a predetermined amount of pressurized air is supplied through the air supplying tube 11a into the left-side chamber space, air pressure in the left-side chamber space is increased so as to move the left piston 2a and piston yoke 2b together to the right. Conversely, when a predetermined amount of pressurized air is supplied through the air supplying tube 11b into the right-side chamber space, air pressure in the right-side chamber space is increased so as to move the right piston 2a and piston yoke 2b together to the left. This causes the table 6 to make a reciprocating movement along the length of the cylinder tube 1a.
Although not shown in the drawings, a magnet is provided near the outer periphery of a cylindrical portion of the piston yoke 2b and a proximity switch is provided on the side surface of the cylinder tube 1a. With such magnet and proximity switch, the stroke end of the cylinder can be detected and the reciprocating movement of the piston 2a can be controlled as desired.
The prior art rodless cylinder 1 shows relatively strong load-resistance characteristics against a vertical (Z-axis direction) load moment that is applied to the piston 2a via the table 6. But, the prior art rodless cylinder 1 shows relatively weak load-resistance characteristics against a vertical load that is applied to the piston yoke 2b from the table 6, because, as previously noted, the cylinder 1 has the gap extending along the entire length of the piston movement (X-axis direction) to allow the piston yoke 2b to project outwardly of the tube 1a and to move freely along the tube 1a.
In particular, the rodless cylinder 1 shows extremely weak characteristics against a laterally bending moment that is applied from the center of the piston yoke 2b in the Y-axis direction. The laterally bending moment is such a moment that will rotate the piston yoke 2b in the Y-Z plane. Hereinafter, a moment that will rotate the piston yoke 2b in the X-Z plane will be referred to as a bending moment, and a moment that will rotate the piston yoke 2b in the X-Y plane will be referred to as a twisting moment.
Accordingly, in the case where a robot or the like is constructed using a plurality of the prior art rodless cylinders 1 in such a manner that it is capable of making controlled free movements in two or three-dimentional coordinates space, the total weight of parts and jigs which can safely be placed on the table 6 will be undesirably limited because of the above-mentioned various moments produced due to the weight of the rodless cylinder 1 itself. This makes the robot or the like extremely impractical.
In addition, the prior art rodless cylinder 1 can only detect the stroke end of the piston 2a by means of the magnet incorporated in the piston 2a and the proximity switch, but it can not detect a current position of the stroke of the piston 2a, and so it can not achieve such a function to stop the piston 2a (i.e., table 6) accurately at a desired position (intermediate point) in the stroke. Although the piston 2a can be stopped at an intermediate position in the stroke by applying the equal air pressure to both sides of the piston 2a because air contact surfaces on both sides of the piston 2a are equal in area, stop position of the piston 2a can not be accurately controlled. In particular, in the case where plurality of the rodless cylinders 1 are employed to form two or three-dimensional space and the movement direction of the piston 2a happens to coincide with the gravity direction, it will be extremely difficult to stop the piston 2a (and hence table 6) accurately at a desired intermediate position in the stroke.